In a user interface of an information apparatus, such as a smartphone, a display device and an input device are integrated on an identical surface by overlaying and mounting a touch sensor on a display screen or incorporating the touch sensor in the display screen. User convenience is enhanced by mounting both the display device and the input device over a sufficient area on a small-sized casing. Techniques for a thin speaker called a film speaker have been developed in recent years and overlaying a thin speaker on a display screen has been allowing the speaker to have an area larger than the area of a typical portable apparatus and be mounted without reducing the portability. A large-area speaker is favorable in terms of output of low frequency sound, compared to a small-sized speaker used typically in a portable apparatus. In addition, since such a speaker may be arranged so as to face a user, the reproduced sound is clearer for the user.
In general, the mass of a diaphragm in a speaker is a significant factor for the design of a sound quality. Assuming interfaces such as a screen display, a touch sensor input part, and an interface of a flat speaker are stacked over an identical surface, attachment of a screen protection sticker, an anti-reflection filter, or the like, or adhesion of dust causes the mass of the diaphragm to deviate from a designed value and changes the reproduced sound or frequency characteristics. Also, the reproduced sound varies when the material physical characteristics of the diaphragm, such as Young's modulus or Poisson's ratio, change as the temperature changes or time elapses. The change in the reproduced sound caused by variability in assemble or environment occurs not only in a portable apparatus, such as a smartphone, but also in the case where a film speaker is incorporated in a screen of for example, a wall-mounted television.
For the problem that the audio signal that has been input fails to be reproduced in the speaker as the designed characteristics indicate, deviation from the reproduced sound intended in the design may be minimized by detecting displacement of a diaphragm of a speaker and feeding the displacement amount back to a speaker drive amplifier. For example, Japanese Laid-open Patent Publication No. 2002-243537 proposes a configuration as a vibration detection method suitable for a small-sized microphone, where the displacement of a diaphragm is detected by reference to change in the intensity of transmitted light, which is caused by bending of the diaphragm, after forming in the diaphragm a pattern with a refractive index different from the diaphragm as an optical waveguide.